1. Field of the Invention
This invention relates to process and apparatus for continuous direct casting of metal strip employing a moving chill surface upon which molten metal is flowed for solidification in combination with a tundish or other molten metal receiving vessel which delivers molten metal to the chill surface.
2. Prior Art
The advantages that may be achieved in direct casting of molten metal into thin strip or sheet (hereinafter "strip") on a continuous basis have long been recognized and numerous process and apparatus have been proposed for direct casting of metal strip. It is not believed, however, that any of the prior process or apparatus have been successfully used on a commercial basis, particularly for the production of a high quality, wide strip suitable for use in the as-cast condition for the production of commercial products, or for further processing as by rolling or shaping by other means.
In prior direct strip casting processes employing a continuously driven chill surface which contacts molten metal to be cast, the molten metal is solidified on the chill surface by extracting heat through the chill surface so that a thin skin of molten metal is solidified immediately upon contact with the chill surface. The thin skin increases in thickness as the chill surface moves progressively through or past the molten metal until the strip is completely formed. The thin skin initially formed is bonded or firmly adhered to the chill surface and the bonded contact results in a maximum heat transfer from the molten metal to the chill surface. As the solidifying skin progressively increases in thickness, the extraction of heat results in contraction of the solidifying strip at its bonded interface with the chill surface until the bond is broken, thereby resulting in a substantial reduction in the rate of heat extraction. The successful production of quality strip by the foregoing process depends to a large degree upon the extraction of heat at a uniform rate to obtain a uniform release of the cast strip from the chill surface. One process for obtaining the required uniform heat extraction through the chill surface is disclosed and claimed in copending U.S. patent application Ser. No. 155,710, assigned to the assignee of the present application, which involves establishing a natural oxide layer on the chill surface and maintaining the natural oxide interface in a smooth layer of substantially uniform thickness. The natural oxide layer is maintained in the required condition by engaging and polishing the natural oxide layer which is formed as a result of exposure of the chill surface to atmosphere. The polishing is effective only to remove the outermost particles of the oxide layer while leaving a packed layer of natural oxide firmly adhered to the chill surface.
Efforts to produce direct cast strip in commercially acceptable widths have revealed problems which are not encountered in the production of more narrow strips on laboratory or experimental apparatus. From a practical standpoint, a chill surface on which the strip is solidified must be substantially wider than the width of strip to be cast and in any commercial installation, the capital cost dictates that the casting wheel and other apparatus be capable of operation to produce strip of various widths. Cooling water circulating through a casting wheel will cool the portion of the chill surface adjacent each end of the casting wheel which does not contact the molten metal during operation which, in turn, will reduce the temperature of the chill surface in contact with the edge portions of the strip being cast. This results in more rapid cooling at the strip edges and can produce an increased thickness at the edges and a reduction of strip thickness near the edge. Such phenomena produces non-uniform strip cross section and relatively large edge thickness sometimes referred to as a "dog bone" shape.
Another problem encountered in producing acceptable strip of commercial widths results from the inherent tendency of molten metal to channel, or flow at non-uniform rates through the tundish or molten metal supply vessel (hereinafter, tundish) with the result that, in areas of most rapid flow, the temperature of the metal reaching the chill surface is higher than in the areas of slower flow. Temperature variations of the molten metal contacting the chill surface is manifested in strip thickness variations, and this problem tends to increase with increased strip width.
Numerous tundish designs are disclosed in the prior art but these known tundish designs, generally, do not recognize the problems in commercial operations and consequently do not suggest any solution to the problems. Typical prior art patents disclosing open tundish designs intended for use in the direct casting of metal strip on a moving chill surface include U.S. Pat. No. 4,715,428; European Patent Application No. 0147912; Swiss Patent No. 626,725; and Japanese Published Application No. 6,035,220. Also U.S. Pat. No. 3,431,971 discloses a tiltable open tundish for continuous casting of metal plate in a rotatable wheel type mold.
Of the above patents, U.S. Pat. No. 4,715,428 is specifically directed to tundish design, and the patent discloses a tundish having an open, generally U-shaped outlet. The tundish gradually decreases in depth and increases in width from its inlet to its outlet, and the patent suggests that plates 36, partially submerged in the molten metal, may be employed to facilitate development of uniform flow. These plates are used in baffling or dampening the flow to obtain uniformity of flow across the full tundish width and to restrain movements of surface oxides and slag. It is not suggested, however, that the plates 36 can reduce channeling or the effect of temperature at the tundish outlet.
It is accordingly, a primary object of the present invention to provide a novel tundish structure for use in the direct casting of thin metal strip.
Another object of the present invention is to provide a novel tundish structure for containing and supplying molten metal to a moving chill surface for producing a strip of commercially acceptable widths and of substantially uniform thickness throughout its width.
Another object of the invention is to provide a novel tundish structure for containing a supply of molten metal and for conducting the molten metal by gravity flow into contact with a moving chill surface in a manner to present molten metal to the chill surface at a substantially uniform temperature throughout substantially the full width of the strip being cast.
Another object is to provide a tundish which is economical to construct and maintain and which is reliable in operation and service.